1. Field of the Invention
The present invention relates to a sample for transmission electron microscope analysis, and a method for manufacturing the sample. The sample is formed as a thin piece, and is used to analyze a defect occurring in a predetermined portion of a semiconductor device using a transmission electron microscope ("TEM"). The sample and its manufacturing method are designed to prevent a possible electron charging during an ion-polishing step and during the TEM analysis, by forming a conductive material on the sample before the sample is subjected to focused ion beam (FIB) polishing.
2. Background of the Related Art
A transmission electron microscope is an instrument for analyzing the phase and the composition of a material, by projecting an incident electron beam accelerated with high potential on a thin sample (below about 1,000.ANG. in thickness). TEM analysis is highly applicable to a sample of about 10 .mu.m or less, and this thin sample can be manufactured only by a focused ion beam system (FIB).
A disadvantage of the TEM analysis of the prior art is that it is impracticable for use on a sample from a substrate that is an insulating material, or a semiconductor containing an insulating layer such as a liquid crystal display (LCD), because the sample becomes charged during the manufacturing procedures and during the TEM analysis.
The above-mentioned electron charging occurs during the TEM analysis as follows. Electrons projected on the sample permeate the sample, which results in back scattering and secondary electron emission. In all cases, the numerous incident charges lose their original energies and are captured in the sample. In the process, if the sample is conductive and has a ground path, the charges will flow away through them. If the sample, however, includes an insulating layer without any grounding path, there occurs a rapid charging even in a conductive sample so that the surface potential of the sample increases.
When the sample captures charges sufficient to raise its surface potential above a predetermined level, a phase to be analyzed cannot be obtained because the charges serve as electron mirrors which reflect the incident electron beam out of the sample. Further, the surface potential of the sample periodically varies due to the repeated emission and accumulation of secondary electrons.
Some have employed a method of depositing a conductive layer on the region to be analyzed in order to prevent the charging described above. But, this method is not applicable to TEM analysis, which analyzes an image obtained on a thin sample projected by an electron beam. This is because, when a TEM sample contains a conductive material, the actual image or phase of the sample overlaps with the image of the conductive material on the sample.
The TEM analysis is, therefore, practicable for a conductive sample with a ground path, but not applicable to the analysis of a sample containing an insulating layer or an insulating sample.
Despite these limitations on TEM analysis, it has become increasingly necessary in recent years to carry out TEM analysis for the microscopic region of semiconductor devices such as memory devices or the like, particularly an LCD whose base layer is made of glass (SiO.sub.2).
In addition, the above-described charging effect on an insulating sample has a deleterious effect on the FIB grinding process in manufacturing the sample itself, by causing an over-etching or under-etching at the analysis point of the sample.